Ellen, like Ben Franklin was fond of saying, "In theory, practice and theory are the same. In practice they are not". BoB is basically saying the same thing in the 2 statements; an electrical filter is relatively easy to make, and is typically very close to specs. So a typical 12dB xover at 80 hz would be down 12dB at half and double the center freq, ie 40 and 160 Hz. A sealed box sub would have to have a B2 response, ie Qts=.707, to properly interact, which in reality doesn't always happen. But wait, there's more. The real problem lies in the room interaction; if you did a top-notch analysis of your rooms response, you'd likely be horrified. My room has dips of 8-10 dB, and peaks of 10 at 55, and 18dB at 120 Hz. Even after a bass trap, which causes my wife, and therefore myself no end of grief, and equalization, I still have +/- 5 dB through the bass range. So IMNSHO, the exact response of speaker/xover is less critical than theory might suggest.
Mike